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Vitamin D supplements do not prevent cancer

Vitamin D has recently become a trendsetter in the supplement crowd, with miracle claims for everything up to and including cancer. Of course, these pro-supplement people think it prevents COVID, but no, it doesn’t. They claim that vitamin D boosts the immune system, but no, it doesn’t. They claim that it reduces blood pressure – no again. I could go on and on, but the wild claims about the effectiveness of vitamin D supplementation on human health have been vastly exaggerated.

Recently, I’ve seen claims that vitamin D either prevents or, shockingly, cures cancer. I mostly dismissed with a wave of my hands any claims about this or that substance or food prevents or cures cancer that aren’t published in a biomedical journal. And even then, I am skeptical of anything published that doesn’t include data from a large clinical trial or cohort study.

I’m going to review some of the research regarding vitamin D for the prevention (or curing) of cancer. Spoiler alert, there isn’t anything.

different medicines placed on white surface vitamin d cancer
Photo by Pavel Danilyuk on

What is vitamin D

Vitamin D is a group of fat-soluble secosteroids (a scientific name for steroids with a “broken” ring). The most important chemicals in this group are vitamin D3 (known as cholecalciferol) and D2 (known as ergocalciferol).

Very few foods contain either of the important types of vitamin D. However, some foods can be good sources of the vitamin:

  • Fatty fish, like tuna, mackerel, and salmon
  • Foods fortified with vitamin D, like some dairy products, orange juice, soy milk, and cereals
  • Beef liver
  • Cheese
  • Egg yolks

Many people with broad diets that include fish, eggs, and other foods can get sufficient amounts of the vitamin without supplementation. As I’ve repeated often, short of chronic malnutrition, we get plenty of it in a normal, healthy diet.

Moreover, vitamin D is produced by a process called dermal synthesis. That is, sunlight, specifically UV-B radiation, causes the synthesis of vitamin D in the skin. Technically, vitamin D isn’t a vitamin, because we can manufacture it, it is a hormone. For this article, we’ll just call it a vitamin, even though biologically it is not a vitamin.

Although we can manufacture sufficient vitamin D by sunbathing every day, the body has a feedback loop that shuts down production to prevent toxicity. Yes, excess vitamin D is quite dangerous leading to many conditions such as over-absorption of calcium which can lead to hypertension and fatigue. It also can lead to some dangerous chronic conditions that we’ll discuss later.

Although humans can manufacture the vitamin by sitting in bright sun, there’s one major problem – the risk of skin cancer. As I’ve written before, there are very few ways to prevent cancer, but staying out of the sun is one of them.

Finally, the vitamin D we consume or produce in sunlight is not biologically active. It is generally activated by enzymatic conversion (in a process called hydroxylation) in the kidneys and liver so that the body can use it.

I need to make some points clear:

  1. Some people have been medically diagnosed with vitamin D deficiencies and require supplements to maintain a proper blood level.
  2. Some medical conditions require vitamin D supplementation. For example, people who get bariatric surgery may require vitamin supplements including vitamin D.
  3. Just because a little vitamin D may prevent certain medical conditions, it does not mean excess amounts will be healthier. The body needs a certain amount of vitamin D, and that’s it. More isn’t going to do more.
  4. Excess vitamin D is toxic.

Vitamin D and cancer

I’ve reviewed some of the published medical literature on the effects of vitamin D on various cancer diagnoses. As I said above, the evidence is lacking.

  • Does vitamin D prevent breast cancer? No evidence, no evidence, no evidence, no evidence, and no evidence. Essentially, all of these very large studies that examined potential links between vitamin D levels and supplementation found no correlation with the risk of breast cancer.
  • Does the supplement reduce the risk of colorectal cancers? No evidence.
  • Does the supplement improve cancer outcomes? Meta-analysis says no.
  • Does vitamin D reduce the risk of all cancers? A large placebo-controlled study of nearly 26,000 participants with no history of invasive cancer or cardiovascular disease enrolled in the trial. The researchers found that supplementation with vitamin D did not result in a lower incidence of invasive cancer or cardiovascular events than placebo. Those with higher blood levels of vitamin D also did not show any decrease in risk of any invasive cancer.


A lot of research that seems to support the link between vitamin D and cancer is from cell culture or animal model studies. There’s an old joke that scientists have “cured cancer over 100,000 times in mice.” The fact is that less than 1% of animal or cell culture studies ever have clinical relevance or get approved by the FDA.

Just a suggestion, if you see someone promoting a cure for cancer in mice, tell that person that you won’t hold your breath until a clinical trial supports their claims. In other words, roll your eyes at animal studies.

Right now, large studies have shown that vitamin D does not affect the incidence of individual cancers or all cancers. There is a possibility, out into the future, that some researchers might find that vitamin D levels are linked to the risk of say liver cancer or bone cancer. But these studies I’ve listed just don’t show us even a hint that vitamin D levels or supplements have anything to do with any cancer.

Once again, if you have been diagnosed with lower levels of vitamin D, follow your physician’s advice in taking a supplement. But don’t expect that if you take more of it you are going to be cancer-free for the rest of your life.


Michael Simpson

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